Variables act as placeholders for data. They allow us to store and reuse values in our program.

# Variable 'x' stores the integer value 5
x = 5
# Variable 'name' stores the string "Sam"
name = "Sam"  

Rules for Naming Variables

To use variables effectively, we must follow Python’s naming rules:

• Variable names can only contain letters, digits and underscores (_).
• A variable name cannot start with a digit.
• Variable names are case-sensitive (myVar and myvar are different).
• Avoid using Python keywords (e.g., if, else, for) as variable names.

Examples

Valid Example

age = 21
_colour = "lilac"
total_score = 90

Invalid example

1name = "Error"  # Starts with a digit
class = 10       # 'class' is a reserved keyword
user-name = "sam"  # Contains a hyphen

Basic Assigment

x = 5
y = 3.14
z = "Hi"

Dynamic Typing

x = 10
print(type(x))
x = "Now a string"
print(type(x))

Multiple Assignments

a = b = c = 100
print(a, b, c)

x, y, z = 1, 2.5, "Python"
print(x, y, z)

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Type casting

Casting refers to the process of converting the value of one data type into another. Python provides several built-in functions to facilitate casting, including int(), float() and str() among others.

Basic Casting Functions

• int() – Converts compatible values to an integer.
• float() – Transforms values into floating-point numbers.
• str() – Converts any data type into a string

# Casting variables
s = "10"  # Initially a string
n = int(s)  # Cast string to integer
cnt = 5
f = float(cnt)  # Cast integer to float
age = 25
s2 = str(age)  # Cast integer to string

# Display results
print(n)  
print(cnt)  
print(s2)

Getting the Type of Variable

In Python, we can determine the type of a variable using the type() function. This built-in function returns the type of the object passed to it.

n = 42
print(type(n))

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Scope of Variables

local Variables

Variables defined inside a function are local to that function.

def f():
    a = "I am local"
    print(a)

f()
# print(a)  
# This would raise an error since 'local_var' is not accessible outside the function

Global Variables

Variables defined outside any function are global and can be accessed inside functions using the global keyword.

a = "I am global"

def f():
    global b
    b = "b is also globally available"
    print(a)

f()
print(b)

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Undertstanding assigment

x = 5

When x = 5 is executed, Python creates an object to represent the value 5 and makes x reference this object.

Now, if we assign another variable y to the variable x.

y = x

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del variable

We can remove a variable from the namespace using the del keyword. This effectively deletes the variable and frees up the memory it was using.

# Assigning value to variable
x = 10
print(x) 

# Removing the variable using del
del x

# Trying to print x after deletion will raise an error
# print(x)  # Uncommenting this line will raise NameError: name 'x' is not defined